1. Field of the Invention
This invention relates generally to a method and apparatus that provide linear actuation effected by the controlled inflation of a deployable tube. The invention relates more specifically to a method and lightweight apparatus that provide linear actuation for use in the deployment of very lightweight space structures such as solar arrays, reflectors, sunshields, radar arrays, antennas, and concentrators.
2. Description of Related Art
Most conventional methods for deploying a space structure accomplish the deployment by means of deployable truss structures consisting of relatively heavy elements such as rigid members, hinges, latches, and cables. Increases in the number of satellites to be launched over the next several decades, however, will emphasize the need for the reduction of space hardware mass, stowage volume, and cost. One approach to realizing these reductions is through the use of inflatable, deployable, space structures. Inflatable structures offer many benefits over conventional deployable structures because they are lower in mass and can be packaged into small volumes, which reduces launch vehicle size and cost. The performance benefit margin of inflatable structures increases as the size of the structure increases, thus making the technology more attractive for large-scale systems. Examples of satellite components that benefit from the utilization of inflatable structures include solar arrays, communications antennas, radar antennas, thermal/light shields, and solar sails.
A critical component of the inflatable structure is the means for control of the deployment process. The deployment of inflatable structures in space to date has demonstrated the need for improvements in deployment control. For example, inflatable space structures have been used in space since the late 1950""s when the first balloon satellites were flown. Balloon satellites, such as the 30 m diameter ECHO series, were deployed from a 0.67 m diameter packing container using inflation gas. The deployment event of the balloon satellites was uncontrolled and depended only on the packing method used. Over thirty-two launches of inflatable spheres occurred during the period from 1958 to 1971 with some remaining in orbit for over eleven years. Several of the early balloon satellites failed during the inflation event. Some of these failures were attributed to lack of control of the inflation process. Modifications were made to the packing and inflation procedure which lead to success with subsequent flights. This was the first experience with design modification to ensure deployment success. Balloon satellites, by nature of their spherical geometry, facilitated this type of a solution to obtain a controlled deployment.
The controlled deployment of inflatable structures having more complex geometric shapes such as beams and toroids has required more elaborate solutions. In 1996, NASA and the Jet Propulsion Laboratory flew the Inflatable Antenna Experiment (xe2x80x9cIAExe2x80x9d) on STS-77. This experiment was conceived to verify the accuracy of an inflatable off-axis parabolic lenticular antenna structure deployed in space. The system consisted of a 14 m lenticular, supported around its perimeter by an inflatable torus. This assembly was attached to the parent spacecraft by three 28 m inflatable struts. The IAE was packed with the struts z-folded and packaged between the folded lenticular structure and a kick-plate. Once the outer doors were opened and verified in place, a command was sent to the kick-plate that was to exert an impulse force on the packed structure. While it had been envisioned that this impulse force would extend the structure to approximately 90% of its length, thus facilitating a linear inflation path. the packed inflatable structure instead extended out of the container and away from the kick-plate. This phenomena was attributed to residual inflation gas within the structure that caused it to auto-inflate and begin the unfolding of the assembly. Another cause of this phenomena was the residual stress in the membranes at each of the packaging folds. This stress caused the z-folds to open slightly, in the same manner as a spring, and move the package away from the kick-plate. Therefore, once the kick-plate was fired, the packed inflatable was no longer resting against it, and no effect was noted.
The loss of the impulse force input and subsequent extension of the structure led to an uncontrolled deployment. During the deployment the spacecraft was pitched in various directions but returned to near its original orientation when the inflation was complete. This deployment demonstrated that significant impulse forces could be imparted to the spacecraft and that the inflatable could violate set zones of exclusion around the spacecraft if the deployment was not controlled.
Thus, to achieve a controlled deployment, it is necessary to control the rate of deployment, control the directionality of deployment, and maintain a uniform internal pressure in the structure during deployment. Control of the rate and smoothness of deployment is important in order to limit impulse forces and moments which may be imparted to the parent craft or the device being deployed. Control of the rate is also important because changes in the rate will yield large volume changes which affect the internal pressure and thus the system rigidity during deployment.
Control of the directionality of deployment is of particular importance with inflatable structures because without strict control, their path during inflation can be random and chaotic. The means for control must dictate the path of deployment and ensure that zones of exclusion around the spacecraft, such as, for example, areas populated by solar arrays or instruments, are not violated.
Successful operation of an actuation apparatus depends on maintaining the stiffness of the inflatable tube while the tube is being deployed. Because the rigidity of the structure is derived from the tensile stress in its walls, and is proportional to the internal pressure, maintaining a pressurized column of gas from the source of inflation to the point of deployment control will maintain rigidity during deployment. To achieve this rigidity, however, it is necessary to provide sufficient resistance to deployment such that the internal pressure required to continue deployment is high enough to adequately stiffen that portion of the tube which has already been deployed.
Therefore, a general need exists for a method of linear actuation effected by the inflation of a deployable tube. A more specific need exists for a method and lightweight apparatus capable of facilitating the actuation by controlling both the rate and the directionality of deployment, and by maintaining a uniform internal pressure in the structure being deployed.
As indicated above, most conventional methods for deploying and supporting a space structure accomplish the deployment by means of deployable truss structures consisting of relatively heavy elements such as rigid members, hinges, latches, and cables. increases in the number of satellites to be launched over the next several decades, however, will emphasize the need for the reduction of space hardware mass, stowage volume, and cost.
Inflatable structures offer many benefits over conventional deployable structures because they are lower in mass and can be packaged into small volumes, which reduces launch vehicle size and cost. The performance benefit margin of inflatable structures increases as the size of the structure increases, thus making the technology more attractive for large-scale systems. Examples of satellite components that benefit from the utilization of inflatable structures include solar arrays, communications antennas, radar antennas, thermal/light shields, and solar sails.
Although inflatable tubular structures weigh less than deployable truss structures consisting of elements such as rigid members, hinges, latches, and cables, the weight of the inflatable tubular structures is not insignificant.
Therefore, a need exists for a method and an apparatus capable of facilitating the deployment, and subsequent support, of a space structure, but which can do so with an overall apparatus weight which is less than that of conventional inflatable deployment structures.
It is an object of the present invention to provide a method and apparatus that provide linear actuation effected by the inflation of a deployable tube. It is a further object of the present invention to provide a method and lightweight apparatus capable of facilitating the actuation by controlling both the rate and the directionality of deployment, and by maintaining a uniform internal pressure in the structure being deployed.
Accordingly, the present invention advantageously employs a lightweight actuation apparatus that permits the deploying end of a tube to be guided out in a controlled, linear motion by a yoke that bears on, or constrains, the inflated portion of the tube immediately behind the apparatus.
The apparatus comprises a frame assembly that is attached to a first, deployable end of an inflatable tube, and a fixed second end of the inflatable tube that is attached to a mounting surface, typically a spacecraft bus or satellite. Inflation gas is introduced into the tube at its fixed second end. As the tube inflates, it deploys in a linear manner so as to extend its length. The actuation apparatus allows the deploying end of the tube to extend while it is guided out in a controlled linear motion by a yoke that bears on, or constrains, the inflated portion of the tube immediately behind the apparatus.
The frame assembly can be configured to interface with a variety of components to be linearly deployed. Typically, this component to be deployed is one or more membranes that comprise the space structure, which may be, for example, a solar array, reflector, sunshield, radar array, antenna, or concentrator.
The apparatus is compatible with inflatable tubes made of various rigidizable materials such that, if desired, the inflated tube, once deployed with this apparatus, can be rigidized and made self-supporting without internal pressurization.
The advantages associated with the present invention are numerous. First, as indicated above, most conventional methods for deploying a space structure accomplish the deployment by means of deployable truss structures consisting of heavy elements. The present invention, however, provides a lightweight means for both deploying and, if desired, supporting a variety of space structures. In a preferred method of actuation, the apparatus allows the unrolling motion of an inflating tube to accomplish linear actuation on the membranes to be deployed, thus avoiding the need for the membranes themselves to be rolled up with the tube in order to be deployed. By virtue of its inflatable means for deployment and optional rigidizable means for support, the invention comprises a lighter-weight system with fewer moving parts, thereby providing a higher degree of system reliability.
Thus, while specifically facilitating as its primary application the deployment of very lightweight space structures, the invention also more generally provides for the deployment of any lightweight structure which can then be maintained in place by the rigidity of the inflatable tube.